That is not exactly correct. You don't need expensive sensors to synchronize maps with reality. The coarsest estimate of where walls are can be used to infer a position in a map; it will just not be very precise. But errors of cms should be ok.

And Roomba only moves in a plane as a result of its (slow) actuators, whereas an iPhone X tries to do hyper-accurate 6dof real time position, from a very quick external source (your hand), for augmented reality. Thus the sensor and computation cost premium. But that does not apply to the Roomba.